1. Field of the Invention
The present invention relates to an equalizer and an audio device using the same, and more specifically, relates to an equalizer in a portable type audio device (such as a portable type magnetic tape reproducing device) which is composed of a circuit constituted by a limited number of elements, is capable of sound quality control and is suitable for forming into an integrated circuit.
2. Background Art
FIG. 6 is an equalizer in a conventional audio device. The equalizer is generally constituted by simulating equivalently of resistors and capacitors through a gyrator circuit provided with a multiplicity of variable Gm amplifiers (in the present example, three variable Gm amplifiers Gm1, Gm2 and Gm3) as illustrated. In the drawing, a control circuit which controls the current for the variable Gm amplifiers is omitted.
Since a multiplicity of transistors are used for constituting one variable Gm amplifier, when forming such an equalizer into an integrated circuit, an increase of the circuit scale cannot be avoided.
When such an equalizer is used in a portable type audio device, it is difficult to form the equalizer into an integrated circuit together with other circuits and, further noises is likely generated because of an increase of circuit passages. An increase in the number of integrated circuits causes an obstacle with respect to the cost thereof as well as an obstacle with respect to the downsizing and thickness redution thereof.
For this reason, an equalizer constituted by a combined circuit of a filter circuit including a capacitor and an amplifier circuit is generally used for the portable type audio device, and moreover in such an equalizer is employed a constitution in which frequency characteristics can be selectively changed-over by a switch.
FIG. 7 is an example of a stereo audio device that is driven by a battery, in which an audio amplifier 20 is primarily illustrated. An output circuit 10 is constituted by an output amplifier 11 for left channel (L side) composed of an operational amplifier and an output amplifier 12 for right channel (R side) composed of an operational amplifier which are for reproducing sounds at right and left sides and a voltage follower type center amplifier 13 which generates a reference voltage of about Vcc/2 and produces a virtual grounding condition. The amplifier characteristics of these amplifiers normally show a substantially flat frequency characteristic in an audio band.
The output amplifier 11 at the L side receives output signals from a low frequency region mixing amplifier 15 composed of a differential amplifier via a variable resistor 14. Similrly, the output amplifier 12 at the R side receives signals from a low frequency region mixing amplifier 17 composed of a differential amplifier via a variable resistor 16.
The respective low frequency region mixing amplifiers 15 and 17 receive from respective preamplifiers 18 and 19 at L and R sides reproduced audio signals at respective (+) input terminals and at the same time receive outputs from a bass boosting amplifier 21 at respective (-) input teminals via respective buffer amplifiers 15a and 17a.
The bass boosting amplifier 21 receives a sum of L+R signals of both channels at L and R sides from the preamplifiers 18 and 19 via a low pass filter 22 composed of a resistor, a capacitor (RC), and amplifiers, and outputs the low frequency region components of the L+R signals. Therefore, the amplification rate at the low frequency region of the low frequency region mixing amplifiers 15 and 17 increases depending on the output of the bass boosting amplifier 21.
Symbols Ca, Cb, Cc and Cd are respectively coupling capacitors and numerals 23 and 24 are right and left side headphones connected to the respective amplifiers in the output stage 10. A dry battery used as a power source is omitted from the drawing.
In comparison with an audio device such as a component stereo which generally has an ample margin or only a slight limitation on circuit size and power source voltage, a portable type audio device driven by a lower voltage must sacrifice sound quality which has been allowed until now. However, higher sound quality to as well as a trend toward multiple functions for such portable type audio device are in demand. The requirement of a high sound quality with the low voltage drive is difficult to realize in a portible device, compared with the general audio device.
When investigating the above explained circuit from this aspect, since the bass boosting amplifier receives the L+R signals and increases the amplification rate at a low frequency region, the separation between right and left channels during bass boosting operation is deteriorated. In addition, since the low frequency region components are amplified by a special amplifier, noise is increased.